Add option to shrotcode for disabling/enabling the auto-select first slot

[Valentina R]

When using multiple shortcodes on 1 page and if the rules are too many, the slots retrieval might hang or become very slow after page load. Having an option to disable the auto-select inside a shortcode would help.

Asked here: https://bookingwp.com/forums/topic/disable-auto-select-only-on-specific-pages/

[Željan Topić]

'availability_autoselect' and 'customer_timezones' are possible attributes for [appointment_form] shortcode from 4.5.1 onward.